The present invention relates to spectrographic instruments such as monochromators, and particularly to a system for correcting astigmatism in such instruments which include spherical off-axis reflectors.
Many spectrographic instruments for dispersing ultraviolet and other electromagnetic light spectra, for example the Czerny-Turner monochromator, include a diffraction grating and one or more spherical reflectors for directing a light beam on a folded path from a light entrance slit to the grating and thence to a light exit slit. Because the reflectors would direct the light back to the light source if it were aligned with their optical axes, the path must be directed to the reflectors at an angle away from each reflector axis. Such off axis reflectors inherently impart astigmatic distortion to the diffracted light image such that the light image at the exit slit is a line rather than a point. With a single light beam at the spectrograph light input a linear spectral image at the output slit can be sensed satisfactorily with a single photodetector. But there is a growing need for simultaneous analysis of spectra of several different point or spot light sources dispersed onto discrete minute areas of a two-dimensional (2D) charge-coupled-device (CCD). Such a spectrometer is described in the article "CCD detectors record multiple spectra simultaneously", Nir and Talmi", LASER FOCUS WORLD, August 1991, 111-119. But the astigmatic line image from a typical monochromator or other off-axis spectrographs tends to overlap several of the minute cells in a 2D CCD blurring the output signals of the CCD even though the several light sources are coupled to the spectrograph entrance slit through separate optical fibers.
One attempt to compensate for the inherent stigmatism of an off-axis spectrograph appears in U.S. Pat. No. 4,932,768 to Gobeli, and involves replacing two of the off-axis reflectors within the spectrograph with mechanically distortable spherical mirrors. Such distortion requires extremely delicate mechanical adjustment of a critical element in the spectrographic optics, with the possibility of misadjustment to which a normal glass reflector of fixed curvature is not subject.
Other systems using CCDs are shown in Photonics Spectra, March 1992, page 55; and Spectroscopy, February, 1991, Vol. 6, No. 2, page 61. These systems produce low quality images however.
Accordingly it is the object of this invention to provide a system compensating for the inherent astigmatism of a spectro-graph with off-axis reflectors, which does not alter the optics of the spectrograph, and which is particularly useful for simultaneous detection of multiple spectra with a two-dimensional charge-coupled-device at the spectrograph output slit.